Thin hafnium oxide film and method for depositing same

ABSTRACT

A thin layer of hafnium oxide or stacking of thin layers comprising hafnium oxide layers for producing surface treatments of optical components, or optical components, in which at least one layer of hafnium oxide is in amorphous form and has a density less than 8 gm/cm 3 . The layer is formed by depositing on a substrate without energy input to the substrate.

FIELD OF THE INVENTION

The invention relates to the field of thin layers of hafnium oxide orstacks of thin layers comprising at least one layer of hafnium oxide. Italso concerns a process for producing thin layers of hafnium oxide.These thin layers are layers appearing in multilayer structuresincorporating, apart from hafnium oxide layers, layers of othercompositions.

PRIOR ART

The deposit of thin layers of hafnium oxide, just like many otherdeposits, is carried out in known fashion by evaporation under vacuum.The basic principle is as follows:

In a vacuum chamber, an electron gun heats a target constituted of thematerial to be deposited. The material is then vaporised and depositedon a substrate, which has been placed close by. In the case of an oxidedeposit, one can also carry out the same operation starting from theproduct to be oxidised, for example a metal, the vacuum chambercontaining oxygen under very low pressure. The latter procedure is knownunder the name “reactive evaporation under oxygen”.

In the case of hafnium oxide, the two techniques are used but forreasons which will be set out below, they are always associated withenergy means such as direct heating of the substrate, ion bombardment ofthe layer during growth, or furthermore acceleration of the ions of thematerial to be deposited by means of an electric field, in such a waythat they provide energy to the substrate.

The deposit processes are described in a more detailed way, for examplein the manual by J. D. RANCOURT “Optical thin film user handbook, SPIEPress, 1996” (C1).

The production of layers of hafnium oxide or hafnia (HfO₂) is usedmainly in multilayer coatings of optical components submitted to highlaser fluxes. The function of the hafnia layers can be to ensureprotection of the components to which they are added.

The component incorporating such layers of hafnia can in itself ensureprotection against the laser flux and the optical function of thecomponent. The multilayer composition can also be deposited as aprotective coating on a pre-existing optical component to attenuate thelaser flux undergone by this optical component. The hafnia layers ensureprotection in the wavelength ranges extending from the ultraviolet tothe infrared for any type of laser, impulse or continuous.

The multilayer components incorporating the hafnia layers ensure theoptical functions for example of mirror, spectral filter oranti-reflection.

The production of these special optical functions requires theproduction of surface treatments. It concerns, by suitable stacking ofmaterials with different refraction indices, creation of aninterferential system, which creates the desired optical function.

In order to do this, two or more materials are used, depositedalternately. In general, two materials, one with a high index and theother with a low index, suffice for the majority of applications.

Many materials can be used to play this role. For example, the followingcan be cited:

-   -   silicon dioxide or silica (SiO₂)    -   alumina (Al₂O₃)    -   titanium dioxide (TiO₂)    -   Yttrium fluoride YF₃)

In the special case of optics submitted to high laser fluxes, the laserdamage threshold (LDT) of stacks is defined as the value of the fluence(or energy received per surface unit) above which appears a permanentmodification of surface treatment. Indications about the modes forproducing layers with high damage thresholds can be found in a book byM. R. Kozlowski “Thin film for Optical Systems” published by F. R. Floryin 1995, (C2). Chapter 17 of this book page 521 and following, providethe present trends in the choice of the material constituting thelayers, the methods for measuring damage and deposit methods. In thischapter, especially in paragraph 3–5 pages 536–537, it is especiallyindicated that the deposit techniques with energy means, as for example,with ion bombardment, have become very widely used because of theimproved possibilities of controlling the thickness and the mechanicalstability of the layer. These processes lead to layers with limiteddefects.

The damage to optical components is the factor limiting the maximumworking laser fluences. This motivates research into improving laserdamage thresholds for surface treatments.

Within this framework, numerous families of materials were studied, forexample, fluoride, chalcogenide and oxide materials. The latter havebeen studied the most within the framework of surface treatmentspresenting high laser damage thresholds.

For applications with high laser flux in the near infrared, theHfO₂/SiO₂ couple is usually used, given the high laser damage thresholdsof the components obtained. Examples of results obtained with thiscouple are described in diverse applications. These publications ingeneral indicate in which manner the layers were deposited. Examples ofsuch publications are given below, for information and for betterunderstanding of what is to follow. An article by Allan J. WARLDORF etAl. published the 1.10.93 in the revue “Applied Optics” Vol. 32 No. 28,pages 5583–5593 (C3) describes the methods and results obtainedconcerning the laser damage threshold with this couple. This articleexplains (in particular page 5583, second column) that the evaporationprocesses by heating the product to be evaporated by heating resistanceor by electron beam give rapid but unsatisfactory results, in particularfrom the point of view of damage thresholds. It indicates that theporosity of the films can be reduced by raising the temperature of thesubstrate, or by acceleration of the ions to be deposited.

It is also indicated that the results obtained with the methods of ionacceleration (ion plating) have encouraged filter manufacturers to usethis method which therefore has tended to be generalised (p. 5584 top ofcolumn 2).

Other articles, for example BRAUN et al. entitled “Optical coating forhigh-power neodymium lasers” published in the review SOV. J. QuantumElectron 18 (10)—1988—page 1286–1290 (C4) or again that of L. L. Chaseet al., “Investigation of optical damage in hafnia and silica thin filmsusing pairs of sub-nanosecond laser pulses with variable time delay, J.Applied Physics, 71(3)(1992), pages 1204–1208” (C5) give damage resultsobtained with refractory oxide layers and in particular with hafniumoxides.

In the two articles quoted above, (C4–C5), the coatings were obtained byelectron beam evaporation. In the second article (C5) it is indicated(page 1204, bottom of column 2) that the substrates were maintained at atemperature of 200° C. during deposit.

In the great majority of published works, the coatings are produced byevaporation under vacuum from a hafnium oxide precursor.

Among the references already given above, on this subject, one canmention documents C2, C4, C5 and also the article by R. J. Tench et al.,“Characterization of defects geometries in multilayer optical coatings,J. Vac. Sci. Technol. A 12(5) (1994) pp 2808” (C6).

This elaboration technique, directly from the oxide, has theinconvenience of generating many defects in the coatings due toprojections issuing from the oxide load during evaporation.

These defects buried in the stack are identified as precursors ofoptical damage under laser flux. Relative to this subject, one canconsult the article of R. Chow et al., “Reactive evaporation oflow-defect density hafnia, Applied Optics. Vol. 32 No. 28 (1993) pp5567” (C7).

There are few publications openly available relative to the productionof components from reactive evaporation of metallic hafnium underoxygen.

Apart from document C7, already mentioned above, it is however possibleto cite the article by J. F. Anzellotti et al., “Stress andenvironmental shift characteristics of HfO₂/SiO₂ multilayer coating.Proceedings of the 28th annual Boulder Damage Symposium, SPIE Vol. 2966(1996)” (C8).

In document C7, the results obtained by the evaporation techniques ofhafnium metal and hafnia are compared.

Although they present attractive results concerning the reduction of thedensity and defects of nodular type, the authors do not demonstrate animprovement of laser damage thresholds for coatings produced by reactiveevaporation of metal under oxygen. It can be thought that the presenceof nodules in the coatings does not constitute a laser damage factoruntil the ultimate performance of the materials has been reached.

Taking into account the state of the art, the inventors considered thatit was possible to obtain good results concerning laser damageresistance, for the production of layers ensuring optical functions forexample of the mirror, spectral filter or antireflection type, from thereactive evaporation of metallic hafnium under oxygen, but on conditionof mastering the growth conditions of the layers in particular theformation of aggregates and crystallinity.

It is known, for example from the article by B. A. Movchan et al. Fiz.Met. Mettaloved, 28 (1969) pp 653 (C9) that the structure and propertiesof condensates of metals or oxides depend closely on the temperature ofthe substrate on which the condensation takes place.

Therefore, the inventors tried to understand better the crystallinegrowth and the formation of metal aggregates and thus the ways to avoidthem.

Non-energy processes with evaporation base produce coatings with highporosity.

In fact, the energy of evaporated species is relatively low: it is afunction of the evaporation temperature of the material but typically ofthe order of several tenths of electron volts (eV). This leads to a verylow mobility of species which condense on the surface of the substrate.In the time interval corresponding to the deposit of two successive atommono-layers, the atom displacements are not sufficient for inducing thecompaction of layers and these vacancies are “buried”. In this way, amicrostructure of porous layers is induced, characteristic of depositsobtained by evaporation. The consequence of this is the production oflayers more difficult to control, for the following reasons:

-   -   appearance of mechanical tension stresses in the layers which do        not favour the stability of the stacks unless specific        compensation mechanisms are put into operation;    -   evolution with time of the optical response of the component        (progressive filling of porosities by water, inducing slipping        of the average refraction index);    -   process drift, the share of the thermal radiation of the        evaporation loads being no longer negligible on a non-heated        substrate. This means that the temperature of the substrate on        which the deposit is made increases, and consequently the        mobility of the species which condense on the substrate.

To minimise or annul the phenomena mentioned above, the great majorityof processes implement means making it possible to densify the coatings.In order to do this, those skilled in the art possess techniquesallowing an energy input on coatings during growth: the heating ofsubstrates and ion bombardment of layers during growth.

As seen above, these techniques, providing the substrate with an inputof energy, are very widely used in evaporation processes for thinlayers.

The inventors have shown that these techniques degraded the performancesof thin layers in terms of laser damage threshold at 1.06 μm.

This idea is based on studies on the damage of thin layers by lasers.Results of these studies have been published by J. Dijon et al., Nanoabsorbing center: a key point in laser damage of thin films. Proc. of28th annual Boulder Damage Symposium, SPIE Vol. 2996 (1996) (C10). It isto be noted that one of the authors of this publication is also one ofthe inventors. The results of these studies are that the presence ofmetallic impurities under aggregate form in the coatings provokes, viathe creation of a relay mechanism of heating of aggregates and radiationin the far ultraviolet, a local transformation of the layers renderingthem absorbent to laser radiation at 1.06 μm, then the coupling of thelaser on the material and destruction of the coating.

It is difficult to avoid the presence of metallic impurities withinsurface treatments whether they are impurities of external origin(material from the evaporation crucible, impurities from the evaporationload, constitutive elements of the evaporation chamber in the case ofutilisation of an ion source) or incompletely oxidised hafnium atoms.

When heating, or which comes to the same from the point of view ofenergy input to the substrate, when bombarding the layer with ionsduring growth, crystallisation of the deposit is encouraged.

It is known that grain boundaries present in a crystallised materialconstitute privileged pathways for diffusion of impurities. An amorphousstructure can “freeze” the diffusion of these impurities.

According to observations made by the inventors, the phenomenon ofcrystallisation of hafnia is necessarily induced by the energy suppliedto the substrate by heating or by ion bombardment. This crystallisationis revealed by peaks of X-ray diffraction. The crystallisationcharacteristic of the energy input is also revealed by transmissionelectron microscopy. With this method the crystalline planes diffractthe electrons in precise directions giving well defined rings from whichit is possible to determine the crystalline structure of the material,for example a monoclinic structure for a deposit carried out on asubstrate heated to 200° C.

BRIEF DESCRIPTION OF THE INVENTION

Taking into account the state of the art which has just been described,the inventors thought that any energy input is unfavourable forwithstanding laser flux since it will encourage the stabilisation ofthese impurities by the formation of aggregates.

The basic idea of the invention, which goes contrary to the usualconcepts of those skilled in the art, consists of working on anon-energy process, that is to say without either ionic assistance orheating of the substrates.

The materials deposited are thus amorphous, that is to saynon-crystalline. In addition, when working with an energy input, onecreates a compaction of the layer deposited. On the contrary, thematerial deposited without energy input has a lower density. In theinventors' documentation no data was found relating to the density ofthe hafnia layers deposited by known processes. The lowest densitiesknown to the inventors are 8.5 gm/cm³. The densities measured on thehafnia layers according to the invention range between 6.4 and 8.1gm/cm³. These densities can be measured by x-ray reflectometry. Aspointed out above, crystallisation only intervenes if the substrates areheated or, which comes to the same thing from the point of view ofenergy input, if the layer is bombarded with ions during growth. Theabsence of crystallisation, according to the inventors, can also explainthe reduction of the formation of aggregates because, as alreadymentioned above, grain boundaries present in a crystallised materialconstitute privileged pathways for diffusion of impurities. An amorphousstructure can “freeze” the diffusion of these impurities.

To summarise, the invention relates to a thin layer of hafnium oxidecharacterised in that the hafnium oxide is in amorphous form with adensity lower than 8 gm/cm³.

Contrary to the layers of hafnium oxides of prior art, an amorphouslayer of hafnium oxide does not show any X-ray diffraction peak.Observation by transmission electron microscopy shows diffuse ringscharacteristic of an amorphous structure. The invention also relates toa stack of thin layers characterised in that it comprises at least onelayer of amorphous hafnium oxide of density lower than 8 gm/cm³ or,furthermore, to an optical component with a surface treatmentcharacterised in that said surface treatment comprises at least onelayer of amorphous hafnium oxide of density lower than 8 gm/cm³.

In general, said layer of amorphous hafnium oxide of density lower than8 gm/cm³ is part of a stack of layers comprising at least one layer witha refraction index different from that of hafnium oxide.

Finally, the invention relates to a process for depositing under vacuumon a substrate at least one layer of hafnium oxide by reactiveevaporation under oxygen of metallic hafnium, said process characterisedin that the deposit is carried out without any energy input to thesubstrate, whether or not this input is before the deposit or during thedeposit.

When one says that there is no energy input, this means that on the onehand the substrate is not heated or pre-heated, and on the other handthat no energy process is used such as ion bombardment of the layerduring growth, or ion acceleration before deposit (ion plating). Thedeposit is a “natural” deposit through the simple effect of condensationof the material to be deposited on the substrate. This deposit is thenamorphous, that is to say there is an absence of crystallinity.

As indicated above, the main advantage provided by layers of amorphoushafnia according to the invention is the very high resistance to laserflux.

The hafnia mono-layer according to the invention, deposited on silica,was able to withstand a laser flux higher than 15 Joules/cm² at awavelength of 1.06 μm with impulses of 3 nsec with a recurrencefrequency of 10 Hz until obtaining 15 Joules/cm². The best prior artknown to the inventors is situated, under the same conditions, in arange from 3 to 5 Joules/cm².

Furthermore, the cold process according to the invention allows a verysubstantial gain in time for the deposit chamber cycle.

In fact, heating the substrates can immobilise the deposit equipment fora whole day when treating the most voluminous pieces, for examplemirrors for the Megajoule laser beam carrier. This mirror will bedescribed below.

With the cold process according to the invention, the production of thistype of mirror requires a time cycle of 5 hours (not including puttingthe chamber under vacuum). It is therefore possible to carry out onedeposit cycle per day, even if it were shown that a cooling ofcomponents between each hafnia deposit significantly improves theperformance of the components.

Finally, with the process according to the invention, it is notnecessary to make deposits on substrates with high thermal stability.

In the production carried out so far, it has not been necessary to coolthe substrate actively, nor to control its temperature in a precisefashion.

For deposits of several mono-layers, the time required is short enoughnot to provoke significant temperature rise; for thicker deposits, twostages were applied (or more if necessary) interrupted by free coolingtime (for example, overnight). It is also possible to envisage coolingtime after the deposit of each layer of HfO₂. These cooling periodstypically last for a time equivalent to one or several deposit times ofthe preceding layer.

It is also possible, in order to reduce the time of utilisation of thechamber for a special mono-layer or multilayer production, to cool thesubstrate actively during growth of the deposit or during aninterruption period of the deposit.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of production of stacks of thin layers among which amorphoushafnia layers according to the invention will now be described withreference to the attached drawings in which:

FIG. 1 shows a cross-section of a substrate carrying a layer ofamorphous hafnium oxide according to the invention.

FIGS. 2 and 3 show comparisons between plots of X-ray diffraction:

FIG. 2 between layers of hafnium oxide comprising oxide crystalsresulting from heating of the substrate and an amorphous layer accordingto the invention,

FIG. 3 between a layer of hafnium oxide comprising crystals of hafniumoxide resulting from ion bombardment of the substrate and an amorphouslayer according to the invention.

FIG. 4 shows a cross-section of a substrate carrying a stack composed oflayers of amorphous hafnium oxide according to the invention and layersof silicon oxide, the stack ensuring a mirror function.

FIG. 5 is a curve representing the reflection of a mirror component ofFIG. 4 in function of wavelength.

FIG. 6 is a partial cross-section of an optical component comprising astack of alternate layers of amorphous hafnia and silicon oxide.

FIG. 7 is a curve representing the optical performances of ananti-reflection component in terms of percentage of transmission oflight in the working spectrum range.

FIG. 8 is a curve representing the values of transmission of a silicasubstrate in the 800–2600 nm range treated with a stack of 10 layers ofHfO₂ and SiO₂ of total thickness 2300 nm.

DESCRIPTION OF EXAMPLES OF PRODUCTION OF STACKS OF LAYERS COMPRISINGLAYERS ACCORDING TO THE INVENTION

FIG. 1 shows a layer of amorphous hafnium oxide 2, according to theinvention, deposited on a substrate 1. It can be checked that thehafnium oxide is amorphous, for example, by observation of the X-raydiffraction plot.

Such plots are shown in FIGS. 2 and 3.

FIG. 2 shows curves 7, 8, 9, 10. The curves represent in arbitraryunits, the intensity of X-rays diffracted depending upon the value ofthe angle of diffraction.

Curve 7 shows the diffraction spectrum of a layer of hafnia producedcold (without heating of the deposit substrate).

Curves 8, 9 and 10 show diffraction spectra of a hafnia layer producedwith a deposit substrate of 100°, 150° and 200° C. respectively.

FIG. 3 shows two curves 11 and 12. These curves show the diffractionspectra of hafnia layers produced either without ion bombardment of thelayer during growth (curve 11), or with ion bombardment (curve 12).

Curves 7 and 11 do not reveal any diffraction peaks 13: the layers ofhafnia are thus amorphous.

On the other hand, curves 8, 9, 10 and 12 reveal diffraction peaks 13characteristic of crystalline materials.

The following examples of embodiments are produced by stacks of layersof HfO₂/SiO₂ for the reasons given above.

The layers of amorphous hafnium oxide are deposited cold according tothe process of the invention, as are the layers of silicon oxide,deposited without deliberate heating nor ion bombardment of thesubstrates.

A first embodiment example relates to the production of a mirror 4 at1064 nm. The mirror 4 must ensure a reflecting function at 1064 nm underan angle of incidence of 45°. This mirror 4 is produced by a stack offormula 12 (HB) H2B where H represents a mono-layer of thickness 156 nmof HfO₂ and B a mono-layer of 213 nm of SiO₂.

A cross-section of this optical component mirror 4 intended to representthe stacking of the layers is shown in FIG. 4. On a substrate 1, themirror according to the invention comprises first of all a stack oftwelve layers H of amorphous hafnium oxide 2, each alternating with alayer B of silicon oxide 3. It then comprises two layers H of amorphoushafnium oxide 2 and finally a layer B of silicon oxide 3.

In order to produce such thick stacks, the radiation of hafnium raisedto more than 2000° C. creates a progressive rise in the temperature bydirect radiation on the substrates. If no special precautions are taken,it is estimated from thermocouple measurements that the temperatureapproaches 100° C. at the end of evaporation. It is possible to reducethis rise in temperature by interrupting the process by periods ofcooling. In this embodiment the reflecting layer was obtained in twostages with a phase of natural cooling half-way through the stacking.FIG. 5 shows the reflection of the component in function of wavelength.Reflection reaches 99.5% at 1200 nm. Measurement at an angle ofincidence of 45° gives a reflectivity higher than 99% at 1064 nm. Adamage test was carried out with increasing fluence according to the “ROn 1” method described on page 523 of document C2. According to thismethod, each test zone is illuminated with increasing series of severalhundreds of impulses, here of 3 nanoseconds each, with a recurrencefrequency of 10 Hertz, of increasing fluence. The average damagethresholds measured are of the order of 50 Joules/cm², that is to say avalue much higher than any mentioned in present publications. ThusWaldorf et al. report in document C3 that they obtained values ofthreshold laser damage at 1.06 μm lower than 2 Joules/cm² from hafniumevaporation, but with ion bombardment, (species energy Ar⁺ and O₂ ⁺which bombard the substrate during growth of the order of 40 eV in thisstudy) thus energy input to the substrate which results incrystallisation of the hafnium oxide contrary to the process of theinvention which itself is without energy input and results in layers ofamorphous hafnium oxide.

The inventors consider that these good results are due to the absence ofmetallic aggregates and/or the amorphous form of the hafnia layersaccording to the invention whereas on the contrary, according to priorart, the energy input resulted in crystallisation of the material.

The second example is relative to a surface anti-reflection treatment inthe wavelength range of 550 to 670 nm. This treatment can, for example,be used as treatment for an optical component in a SILVA laser chainused for the isotopic separation by laser of uranium in vapour phase. Astack of six layers of HfO₂/SiO₂ of total thickness 630 nm was depositedon each of the faces of a substrate, the layers of amorphous hafniabeing deposited by the process according to the invention.

In FIG. 6, the optical component 6 is shown, with the aim ofsimplification, with a single example of two-layer compositioncomprising a layer of amorphous hafnia 2 alternating with a layer ofsilicon oxide 3.

The curve in FIG. 7 shows the optical performance of component 6 interms of percentage of light transmission in the working spectral rangeexpressed in nm. It can be seen that the percentage is very close to100% within the entire working range. The behaviour of this componentunder laser flux is shown to be excellent within the utilisationwavelength range.

Thus, the component resisted an irradiance higher than 80 kW/cm² by alaser emitting at a wavelength of 578 nanometers. The frequency ofirradiance pulses, each of 3 ns was 5.6 KHz and the exposure time wasabout five minutes.

The third example relates to the embodiment of a 3-strip antireflectiontreatment. The component described in this example is part of an opticalparametric oscillator device (OPO). This treatment is deposited on acrystal of potassium and titanyl phosphate (KTiPO₄) generally referredto as KTP, which is submitted to laser irradiation at 1064 nm. Theproperties of the crystal are such that this irradiation induces anemission of photons at 1572 and 3292 nm. The aim of the treatment is toeliminate the parasitic reflections at these wavelengths in order toimprove the OPO performances. FIG. 8 shows the transmission values of asilica substrate within the range 800–2600 nm treated with a stack of 10layers of HfO₂ and SiO₂ of total thickness 2300 nm. The representationof this treatment on the KTP crystal would not aid the understanding ofthe invention, and therefore it has not been shown. Measurements ofresidual reflectivity on the KTP crystal are, at the workingwavelengths:

-   -   R=0.1% at 1064 nm    -   R=0.01% at 1572 nm    -   R<7% at 3292 nm

The laser damage threshold at 1064 nm is 23 J.cm⁻², which is remarkablefor a low reflection treatment at this wavelength; in fact thequasi-totality of the flux crosses the component and stresses the stackassembly.

Example of an Embodiment of the Process According to the Invention

Information relating to the process for producing hafnia layersaccording to the invention has already been given above.

For the three examples of embodiments described above, the productionconditions were in compliance with the following table:

HfO₂ layer SiO₂ layer Crucible Without crucible Without crucible or ingraphite Departure material Hf metal turnings SiO₂ granules (1–3 mm)Electron gun 10 kv ≅ 400 mA 10 kV ≅ 200 mA O₂ pressure 4.10⁻⁴ mbarwithout O₂ addn. Deposit rate 0.25 nm/sec 0.45 nm/sec Deposit temp.ambient ambient

There is a significant difference from prior art: as can be seen, thetemperature of deposit substrate is equal to the ambient temperature,that is about 20° C. and not 200° C. as in prior art.

1. A thin layer material consisting essentially of amorphous hafnium oxide having a density less than 8 gm/cm³.
 2. A stack of thin layers, said stack including at least one layer consisting essentially of amorphous hafnium oxide having a density less than 8 gm/cm³.
 3. The stack of thin layers as claimed in claim 2, wherein the stack comprises at least one layer of another material formed on a surface of the amorphous hafnium oxide layer.
 4. The stack of thin layers as claimed in claim 3, wherein said another material comprises silicon oxide.
 5. The stack of thin layers as claimed in claim 2, wherein the stack comprises alternate layers of amorphous hafnium oxide having a density less than 8 gm/cm³ and another material.
 6. The stack of thin layers as claimed in claim 5, wherein said another material comprises silicon oxide.
 7. An optical component having on at least one surface at least one layer consisting essentially of amorphous hafnium oxide having a density less than 8 gm/cm³.
 8. The optical component as claimed in claim 7, and comprising a stack of said thin layers of amorphous hafnium oxide.
 9. The optical component as claimed in claim 8, wherein the stack comprises alternate layers of said layers of amorphous hafnium oxide having a density less than 8 gm/cm³ and layers of another material.
 10. The optical component as claimed in claim 9, where said another material comprises silicon oxide. 